The present invention relates to a method of growing a ZnO film using chemical vapour deposition (CVD), and to a ZnO film grown according to the method.
Zinc oxide is a material which, when prepared as a polycrystalline thin solid film, has a variety of useful applications, including gas sensors, surface acoustic wave (SAW) devices, optical modulators, optical waveguides, and piezoelectric actuators for micro-machines. The wide range of applications stems from the fact that zinc oxide is a semiconductor with a wide optical bandgap (3.3 eV) and has piezoelectric and electrooptic effects which can be exploited. The magnitudes of the electrooptic and the piezoelectric effects in polycrystalline films of zinc oxide tend to be less than in single crystal zinc oxide, but can be maximised by growing a film such that the grains have a preferred crystallographic orientation, such as the (0001) orientation.
A variety of different techniques have been used to grow thin solid films of zinc oxide, including chemical vapour deposition (CVD), spray pyrolisis, reactive evaporation, and sputtering. Of these techniques, CVD is gaining popularity as it allows films to be grown on substrates with non-planar surfaces, and is a low energy process which causes minimal damage to the substrate surface. CVD involves reacting a vapour of one or more precursors at a surface of a heated substrate such that stoichiometric crystals of zinc oxide are gradually grown. One prior art CVD method involves growing zinc oxide films from a vapour of basic zinc acetate, but has the disadvantage that it requires the introduction of water vapour to improve the crystallographic orientation and to remove unwanted organic by-products.
According to a first aspect of the present invention there is provided a method of growing a zinc oxide film on a surface of a substrate by chemical vapour deposition, the method comprising the steps of:
a) providing a precursor in vapour form, the precursor substantially comprising Zn4O(O2CNRARB)6, where RA and RB are any combination of alkyl or perfluoralkyl groups;
b) decomposing at least some of the vapour at the surface of the substrate such that the film of zinc oxide forms.
Preferably, the method does not require deliberate introduction of water vapour in order to produce high purity zinc oxide films. Preferably, the number of carbon atoms in the alkyl group is in the range from 1 to 106. Examples of RA/RB groups which can be substituted in the precursor include methyl groups (CH3), ethyl groups (CH3CH2), propyl groups (CH3(CH2)2), isopropyl (CH3CHCH3) etc. they also include any perfluoroalkyl group.
Preferably, the surface of the substrate is heated to an optimal temperature which promotes decomposition of the vapour such that a film of zinc oxide substantially free of impurities forms on the surface. Preferably, the temperature of the substrate surface is low enough to substantially prevent fragmentation of the alkyl or perfluorakyl groups.
In a particularly favourable embodiment, the precursor comprises Zn4O(O2CN(CH2CH3)2)6. In this embodiment, a substrate temperature can be found which makes it unnecessary to deliberately introduce water vapour in order to produce high purity zinc oxide films. It has been found that zinc oxide films grown with this precursor tend to have a piezoelectric coefficient (d33)close to that of single crystal zinc oxide (12 pm/V). The surface of the substrate may be heated in this embodiment to a temperature in the range from 300xc2x0 C. to 600xc2x0 C., more preferably 350xc2x0 C. to 450xc2x0 C. Advantageously, in order to grow a highly pure zinc oxide film from Zn4O(O2CN(CH2CH3)2)6, the surface of the substrate should be heated to a temperature of substantially 400xc2x0 C. The inventors believe that at this temperature the precursor vapour decomposes at the surface of the substrate according to following reaction:
Zn4O(O2CN(CH2CH3)2)6xe2x86x924ZnO+6C2H5NCO+6C2H4+3H2Oxe2x80x83xe2x80x83(1)
The products C2H5NCO, H2O and C2H4, are volatile in vacuum, and thus tend not to be incorporated into the zinc oxide film.
Step (a) of providing a precursor in vapour form may comprise subliming a precursor source material such that the precursor vapour is produced. In one embodiment, step (a) comprises subliming a precursor source material in a Knudsen cell. Walls of the Knudsen cell are heated to a temperature at least as great as the sublimation temperature of the precursor source material, and the Knudsen cell includes at least one aperture for enabling the sublimed precursor to escape from the Knudsen cell in vapour form. For example, if the precursor source material comprises Zn4O(O2CN(CH2CH3)2)2, which has a sublimation temperature under vacuum of xcx9c170xc2x0 C., xcx9c0.1 mmHg the Knudsen cell may be heated to about 190xc2x0 C.-195xc2x0 C.
According to a second aspect of the present invention there is provided a method of growing a zinc oxide film on a surface of a substrate, the method comprising carrying out chemical vapour deposition from a single precursor, the precursor being suitable for decomposing at the surface of the substrate such that a film of zinc oxide forms without deliberate introduction of water vapour.
Preferably, the precursor forms the film during the chemical vapour deposition without deliberate introduction of any other reactant, catalyst, or carrier gas. The precursor of the second aspect may be in accordance with any one of the precursors described in the first aspect of the invention.
The first and second aspects of the invention preferably form a highly crystalline film.
Advantageously, the zinc oxide film grows with a preferred grain orientation that is substantially independent of the atomic structure of the substrate. The film may have a single grain orientation, such as a (001) orientation. The substrate may be arranged to have a crystalline structure and lattice parameter similar to those of bulk zinc oxide in order to promote epitaxial single crystal growth. The surface of the substrate can be planar or non-planar, and can be formed from the silicon, silicon dioxide, copper, platinum, gold, chromium, glass, or any other suitable material.
The substrate of the first and second aspects of the invention may be an optical fibre which is mounted such that the fibre can be rotated during film growth to ensure that the film has a relatively uniform film thickness. The optical fibre maybe pre-coated with one or more other materials, such as an electrode material. The first and second aspects of the invention may include a step of monitoring the temperature of the surface of the substrate in order to control heating of the substrate.
The first and second aspects of the invention are preferably carried out under vacuum conditions, pressures below 10xe2x88x921 Torr are advantageous.
According to a third aspect of the present invention there is provided a solid zinc oxide film grown by chemical vapour deposition from a precursor which substantially comprises Zn4O(O2CNRARB)6, where RA, RB are any combination of alkyl or perfluoroalkyl groups.
According to a fourth aspect of the present invention there is provided a solid zinc oxide film grown by chemical vapour deposition from a single precursor without introduction of water vapour.
Preferably, the zinc oxide film is grown from the single precursor without the introduction of any other reactant, catalyst, or carrier gas.
According to a fifth aspect of the present invention there is provided a substrate having a coating, at least a part of the coating comprising a zinc oxide film grown by chemical vapour deposition from a precursor which substantially comprises Zn4O(O2CNRARB)6, where RA and RB are any combination of alkyl or perfluoroalkyl groups.
According to a sixth aspect of the present invention there is provided a substrate having a coating, at least a part of the coating comprising a solid zinc oxide film grown by chemical vapour deposition from a single precursor without introduction of water vapour.
The substrate of the fifth and sixth aspects of the invention may include an optical fibre, or a portion of an optical fibre.
Throughout this specification, unless the context requires otherwise, the word xe2x80x98comprisexe2x80x99 or other variations such as xe2x80x98comprisesxe2x80x99 or xe2x80x98comprisingxe2x80x99, will be understood to imply the inclusion of a stated element or integer, or group of elements or integers, but not the exclusion of any other element or integer, or group of elements or integers.
An embodiment of invention will now be described, by way of example only, with reference to accompanying drawings.